1. Field of the Invention
The present invention relates to medical electrical stimulation devices. In particular, the present invention is a transcutaneous electrical nerve stimulation (TENS) apparatus.
2. Description of the Prior Art
Chronic pain is one of the most common medical problems encountered by clinicians. It is well known that through the application of electrical pulses to nerves, such as transcutaneous electrical nerve stimulation (TENS), pain can be suppressed. TENS therapy is, in fact, a proven alternative to drug therapy. Investigators have found that 55% to 60% of the chronic pain patients studied obtained satisfactory relief with TENS.
Disadvantages associated with early TENS devices are described in the Background of the Invention section of U.S. Pat. No. 4,431,002. In general, patients were unable to tolerate the unpleasant sensations which accompanied the high intensity stimulation levels necessary to produce long term analgesia. The medical stimulation apparatus disclosed in the U.S. Pat. No. 4,431,002 overcomes these problems to some extent by modulating the intensity and frequency of the stimulation signal. Intensity of the stimulation signal changes as frequency of the stimulation signal varies between two levels. The result is that an increase in stimulation signal intensity due to an increase in frequency is counteracted by a simultaneous decrease in the signal intensity.
The TENS apparatus disclosed in the U.S. Pat. No. 4,431,002 modulates the intensity and frequency of the stimulation signal within preset limits, however. This technique presents several problems. The patient's perception of the stimulation signal will vary with its intensity and frequency. When the stimulation signal frequency and intensity are modulated only between predetermined intensity and frequency limits, the limits must be set at a value which can accommodate the entire range of intensity and frequency settings available. The preset limit is merely a compromise limit, however, and is optimum for only limited intensity and frequency settings. By way of example, when a stimulation signal is being produced at a low frequency of 2 Hz, a 25% modulated change in its intensity would be imperceptible and useless. The same 25% modulated intensity change when the signal has a high frequency of 150 Hz can, however, be intolerable to the patient.
Other problems with prior art TENS devices include the phenomena of accommodation and adaptation. Accommodation is fatigue of stimulus receptors, such as nerves. When a receptor is repeatedly subject to a given stimulus, it becomes accommodated to the stimulus, and its output level decreases. Adaptation is a similar property which occurs within the central nervous system. When the central nervous system is repeatedly subject to the same stimulus from receptors, it begins to ignore the stimulus.
It is clear that there is a continuing need for TENS devices which alleviate the problems associated with perception, accommodation, and adaptation. A TENS device which modulates characteristics of the stimulation signal as a function of a variable parameter, as opposed to within preselected limits, would be especially desirable. The degree to which characteristics are modulated should be adapted to meet the physiology of the human body. Circuitry for implementing these functions should also be relatively inexpensive to make the TENS device cost effective.